Patterned layers of chromium deposited on transparent substrates are widely used as photolithographic masks. For this use, chromium layers are predominately patterned with a negative resist material when fine definition, complex patterns are required. For certain applications, however, chromium layers can be patterned with a positive resist material. There is, of course, a need to increase the resolution of such masks to satisfy the ongoing demand in the electronics industry for more minute, more complex circuitry and devices. To this end, it would be advantageous to be able to produce fine, complex patterns in such chromium layers with a positive resist material instead of the predominately utilized negative resist materials.
A process of utilizing a photoresist material to pattern a chromium layer with a pattern opposite to that in the resist material, i.e. reverse etch the chromium, has been described by Yamazaki et al. in the Japanese Journal of Applied Physics, Vol. 19, No. 7, July 1980, pp. 1374-1376. This process involved doping a chromium oxide layer as it was being deposited onto the chromium. The source of chromium oxide being heat evaporated onto the chromium was contained in a tungsten boat and, hence, tungsten, as its oxide, was carried along as an impurity.
Yamazaki et al. discovered that, when a patterned positive photoresist formed on the chromium oxide was plasma etched, the tungsten oxide impurity underlying the photoresist was removed. As etching continued, both the oxide and the underlying chromium were etched away, but that portion of the chromium oxide not covered by the photoresist was resistant to the etchant. The passivating capacity of the tungsten impurity, therefore, caused the original pattern in the positive photoresist to be reversed in the chromium layer, as if it had been directly patterned with a negative resist material. It will be appreciated that, had the photoresist been negative-working, the pattern produced in the chromium layer would be as if it had been directly patterned with a positive resist material.
In accordance with this invention, there is provided a precise process of reverse etching a chromium layer utilizing wet etching of the chromium which is considerably faster and less expensive than dry etching techniques.